The major components of well-known varistor compositions include SiC, SrTiO3, and ZnO such as ZnO—Bi and ZnO—Pr. Among them, ZnO—Bi-based and ZnO—Pr-based varistors are becoming widespread as surge protectors. ZnO—Bi-based and ZnO—Pr-based varistors, having more excellent voltage nonlinearity and surge current capability than SiC-based and SrTiO3-based varistors, deliver high ability to protect electronic devices from surge currents.
Between these two types of ZnO-based varistors, ZnO—Pr-based varistors exhibit excellent voltage nonlinearity but produce a larger leakage current than ZnO—Bi-based varistors.
To solve this disadvantage, for example, Japanese Examined Patent Application Publication No. 63-30763 proposes a ZnO—Pr-based varistor containing minor components such as praseodymium (Pr); cobalt (Co); at least two of potassium (K), rubidium (Rb), and cesium (Cs); chromium (Cr); and at least one of magnesium (Mg) and calcium (Ca).
To reduce the leakage current, specifically, this varistor contains 0.1 to 5 atm % of praseodymium; 0.5 to 5 atm % of cobalt; 0.05 to 0.5 atm % in total of potassium, rubidium, and cesium; 0.05 to 0.5 atm % of chromium; and 0.01 to 2 atm % of magnesium and/or calcium.
These additives are used in the form of metal oxides or, for example, carbonates, hydroxides, or fluorides, which form oxides by firing.
A ZnO-based varistor positively uses crystal grain boundaries in a ceramic sintered compact. Two crystal grains and their grain boundary provide the minimum unit of the varistor. This varistor, therefore, exhibits excellent voltage nonlinearity. In addition, the voltage nonlinearity can be controlled by, for example, changing the types of the additives used.
However, the voltage nonlinearity decreases by defective grain boundaries, which may increase by, for example, nonuniform precipitation of added elements and the yield of compounds from the added elements. In particular, varistor characteristics such as insulation resistance largely depend on alkali metals precipitated on crystal grain boundaries. For low-voltage varistors containing a small number of crystal grain boundaries, defective grain boundaries generated by the nonuniform precipitation of the added elements and compositional variations impair electrostatic discharge (hereinafter referred to as ESD) characteristics. Such an impairment in the ESD characteristics results in decreases and variations in varistor voltage Vow and insulation resistance IR.
For such a varistor as proposed above, potassium, which is an alkali metal and is added in the form of a soluble potassium compound such as potassium oxide, potassium carbonate, potassium hydroxide, and potassium fluoride, may dissolve in water contained in a first slurry during pulverizing by a wet process. Then, potassium is removed together with water during dehydration, leading to a variation in the composition of the resultant material, that is, a compositional variation. The compositional variation makes it difficult to provide a high-quality varistor having excellent and stable varistor characteristics.
In addition, the dissolution of the potassium compound into water causes a variation in the pH of the first slurry. Such a variation may increase the viscosity of the first slurry to decrease the miscibility, millability and dispersibility of the first slurry. In the first slurry, which is a mixture of solid and liquid, each element is dispersed by electrostatic repulsion caused by the zeta (ζ) potential of the element. The zeta potential depends on the hydrogen ion exponent, namely pH, of the first slurry. If the pH of the slurry varies by the hydration of the potassium compound to a zeta potential of 0 (zero), particles no longer repel each other in the first slurry. Then, the slurry fails to maintain its dispersion system. As a result, the particles aggregate to increase the viscosity of the slurry, leading to decreases in the miscibility, millability, and dispersibility of the slurry.
Furthermore, during drying after the dehydration, potassium readily aggregates and nonuniformly precipitates through recrystallization to decrease the dispersibility. This may result in further impairment in the varistor characteristics.
The present invention is based on the above problems. Accordingly, an object of the present invention is to provide a method for manufacturing a reliable, high-quality varistor that has excellent varistor characteristics such as varistor voltage V1mA and insulation resistance IR and that can prevent variations in these characteristics, and also provide the varistor.